Neurostimulation systems, and increasingly implantable neurostimulation systems, are used to treat various neurological diseases and other neurological disorders, such as epilepsy, movement disorders (e.g., Parkinson's disease) and chronic pain. Research is ongoing concerning use of implantable neurostimulation systems to treat psychological disorders (e.g., depression), headaches and Alzheimer's disease and to facilitate stroke recovery.
A typical neurostimulation system comprises a stimulation source, such as a pulse generator, and a stimulation output (or therapy output) stage through which a form of stimulation (e.g., electric current or voltage) can be delivered to target neural tissue. The output stage is in communication with a plurality of electrodes that are disposed in or near the target brain tissue. For example, a brain lead can be used to connect multiple electrodes located on a distal end of the lead through conductors to a proximal end of the lead which then can be connected to the neurostimulator. The electrode-bearing leads may be designed so that the electrodes are intended to be placed on a surface of the brain (cortical strip electrodes on a cortical strip lead) or within the brain (deep brain electrodes on a deep brain or depth lead).
The stimulation to be delivered to the patient is typically programmable. For example, a neurostimulator may be loaded with a set of programmed instructions that cause it to initiate a stimulation episode according to a particular schedule or in response to some predetermined physiological condition or conditions or a neurological event or events. Various parameters related to the stimulation episodes also may be predetermined by programming, for example, whether the stimulation episode consists of pulsatile or non-pulsatile stimulation, and, if pulsatile, how many pulses in a burst, how many bursts within the episode, and the amplitude, frequency or pulse-to-pulse intervals within a burst all may be programmable. In addition, the electrodes among the electrodes available for stimulation through which a stimulation episode is delivered can be preselected by programming. For instance, there may be four electrodes on the distal end of a deep brain lead, all of which are available for use in a stimulation episode, and the neurostimulation system may be programmable to deliver stimulation for a given stimulation episode in a bipolar fashion from the most proximal electrode on the lead to the next most proximal electrode on the lead, or between the first three most proximal electrodes and the most distal electrode.
As will be appreciated by those with skill in the art, there may be circumstances in which it would be desirable to be able to program a neurostimulation system so that different amounts of current can be delivered through different stimulation electrodes at a given instant, for example, to deliver stronger stimulation to the location adjacent one of the electrodes than the stimulation delivered at the location(s) adjacent the other(s). Thus, what is needed is a device and method associated with a neurostimulation system for independently controlling the current that is delivered through each of a plurality of electrodes available for stimulation.